to where you turn them on and nothing happens. but it is so totally used in every nook and cranny, that making any accommodation to shut it down, to do something to it, is very difficult. narrator: two massive underground tunnels, called simply tunnel 1 and tunnel 2, provide most of the city's water supply. they run hundreds of feet below manhattan, far deeper than the subways. built at the beginning of the 20th century, they are concrete-lined and bored through solid rock. they could last centuries. but the mechanical equipment within them will not. engineers in the 1950s discovered rust on the tunnel's valves. there were concerns that if they closed the valves for tunnel inspections, they may never open again, leaving new york city without water. so they chose to keep them open. as a result, there has not been significant inspection, maintenance, or repair of the tunnels in decades. no one knows their current condition. hurwitz: currently, city tunnel 1 and city tunnel number 2 would be feeding each half of the city. so you'd lose half the city if you didn't have a replacement.